The inorganic biochemistry of molybdenum will be explored by a study of model systems consisting of complexes of the metal in the plus 6, plus 5, and plus 4 oxidation states with oxygen, sulfur, and nitrogen containing ligands of biological relevance. These complexes will be characterized by analysis, electronic, IR, and electron spin resonance spectra, electrochemical properties, and x-ray crystallography. Their reactions with biological ligands (purines, aldehydes, SO3, and NO3) will be investigated by kinetic analysis and appropriate reaction mechanisms will be postulated and tested. The results will be used to interpret the spectra, electrochemistry, metal site structure, and reactions of the molybdenum enzymes.